The present invention relates to an evacuated panel which enables the thermal insulation of a body having non-planar surfaces to be obtained.
Evacuated panels, and particularly those made with plastic materials, are being increasingly used in all fields where thermal insulation at temperatures lower than about 100° C. is required. As examples of such applications can be mentioned the walls of domestic and industrial refrigerators, of drink dispensing machines (where thermal insulation is required, above all, in order to separate the portion for hot drinks, generally at about 70° C., from the portion for cold drinks), or of containers for isothermal transportation, for instance of drugs or cold or frozen food. Further, applications of these panels in the building field or in the car industry are being studied.
As is known, an evacuated panel is formed of an envelope wherein a filling material is provided. The envelope has the function of preventing (or reducing as much as possible) the entry of atmospheric gases into the panel, so as to maintain a vacuum level compatible with the degree of thermal insulation required by the application. For this purpose, the envelope is made with so-called “barrier” sheets, characterized by having a gas permeability as low as possible, which can be formed of a single component but, more frequently, are multi-layers of different components. In the case of multi-layers the barrier effect is conferred by one of the component layers, whereas the other layers generally have functions of mechanical support and protection of the barrier layer.
The filling material mainly has the function of spacing apart the two opposite faces of the envelope when vacuum is created in the panel, in addition to other features which will be pointed out in the following. This material can be inorganic, such as silica powder, glass fibers, aerogels, diatomaceous earth, etc.; or organic, such as rigid foams of polyurethane or polystyrene, both in the form of boards and of powders. The filling material must, in any event, be porous or discontinuous, so that the porosities or the interstices can be evacuated. Since the permeation of traces of atmospheric gases inside the panel is practically unavoidable, these panels also contain, in most cases, one or more materials (generally referred to as getter materials) capable of sorbing these gases, so as to maintain the pressure inside the panel at the desired values.
Because of the rigidity of the materials they are made of, the known panels generally have a planar conformation, and therefore can be used for the insulation of substantially parallelepiped bodies having planar surfaces, but they are not suitable for bodies having curved surfaces, such as bath-heaters or the piping used for oil transport in the arctic regions, or bodies which are provided with superficial depressions and reliefs.
One of the methods used so far for realizing the thermal insulation of bodies having non-planar surfaces consists in connecting together several flat panels in the shape of bands, for example by sticking together the edges thereof by means of a glue, so as to obtain a composite structure which can be bent along the junction lines, so as to adapt it to the shape of the body to be insulated.
However, in these kinds of structures made of planar panels connected to each other, heat transfers take place at the junctions, and therefore the quality of the heat insulation offered by such structures is poor.
International patent application publication WO096/32605 in the name of the British company ICI describes evacuated rigid panels having a non-planar shape and a method for the manufacture thereof, which consists in making grooves in the filling material, prior to the evacuation step, the grooves being arranged in the desired direction and having suitable width and depth. Subsequently, the filling material is inserted into an envelope, and the assembly is subjected to the evacuation step, whereby the panels bend along the grooves and take on the final, non-planar shape. Finally, the evacuated panel is sealed.
However, it has been observed that in the course of the evacuation the envelope adheres to the filling material and becomes at least partially inserted into the grooves so that, when the evacuation is completed, the thickness of the panel is not regular in all the parts thereof, being thinner at the bending lines compared to the planar portions of the same panel. Consequently, the thermal insulation properties are also not uniform, but are reduced along these bending lines.
Another drawback of the known non-planar panels consists in the risk that the envelope, which is pressed inside the grooves, breaks, thus enabling the passage of atmospheric gases toward the inside of the panel, which compromises permanently the properties of thermal insulation of the panel itself.
A further drawback of the known non-planar panels consists in that their curving (bending) is necessarily accomplished during the evacuation step, that is, during the process of manufacture of the panels. The accomplishment of the curving significantly increases the volume occupied by the panels, whereas it would be convenient to carry out the curving at the time of the final application of the panel, so as to reduce the difficulties and the costs for transport and storage thereof.